Causes of Disease
Abnormal hemoglobin, called hemoglobin S, causes sickle cell disease (SCD).
The problem in hemoglobin S is caused by a small defect in the gene that directs the production of the beta globin part of hemoglobin. This small defect in the beta globin gene causes a problem in the beta globin part of hemoglobin, changing the way that hemoglobin works. (See Overview.)
When the hemoglobin S gene is inherited from only one parent and a normal hemoglobin gene is inherited from the other, a person will have sickle cell trait. People with sickle cell trait are generally healthy.
Only rarely do people with sickle cell trait have complications similar to those seen in people with SCD. But people with sickle cell trait are carriers of a defective hemoglobin S gene. So, they can pass it on when they have a child.
If the child’s other parent also has sickle cell trait or another abnormal hemoglobin gene (like thalassemia, hemoglobin C, hemoglobin D, hemoglobin E), that child has a chance of having SCD.
Example of an Inheritance Pattern
The image shows how sickle hemoglobin genes are inherited. A person inherits two hemoglobin genes—one from each parent. A normal gene will make normal hemoglobin (A). A sickle hemoglobin gene will make abnormal hemoglobin (S).
In the image above, each parent has one hemoglobin A gene and one hemoglobin S gene, and each of their children has:
- A 25 percent chance of inheriting two normal genes: In this case the child does not have sickle cell trait or disease. (Case 1)
- A 50 percent chance of inheriting one hemoglobin A gene and one hemoglobin S gene: This child has sickle cell trait. (Cases 2 and 3)
- A 25 percent chance of inheriting two hemoglobin S genes: This child has sickle cell disease. (Case 4)
It is important to keep in mind that each time this couple has a child, the chances of that child having sickle cell disease remain the same. In other words, if the first-born child has sickle cell disease, there is still a 25 percent chance that the second child will also have the disease. Both boys and girls can inherit sickle cell trait, sickle cell disease, or normal hemoglobin.